The use of T1 transmission lines for the electronic transfer of information is based on twisted pair wiring, with separate pairs of wires being used for the transmit and receive sides of the T1 line. Standard T1 transmissions use an alternative-mark-inversion (AMI) format. The AMI format consists of "0" and "1" bits. The "0" bits being represented by the absence of a pulse and the "1" bits being represented by pulses of alternating polarity. The standard T1 transmission frequency for such data is typically at 1.544 megabits per second.
As information is transmitted over a T1 line, the attenuation of the information signal can typically range from zero DB with short lengths of T1 transmission lines to at least 30 DB of attenuation with lengths up to approximately 6,000 feet. Additionally, because of the AMI format of the signal pulses, as the lengths of T1 transmission lines increase the pulses can overlap or smear out over several bit times, this is known as intersymbol interference.
For signals sent over T1 lines with a length of about 100-650 feet, the attenuation, phase shift, and the frequency smearing of the signals caused by the T1 line can be compensated for by providing the signals with a pre-emphasis consisting of controlled amounts of overshoot and undershoot.
However as the length of the T1 line increases, the amount of attenuation, phase shift, and smearing of the signals becomes too great to be compensated for by signal pre-emphasis. As discussed above, as the length of the T1 transmission line approaches 6,000 feet, the transmitted pulses become smeared out over several bit times such that the pulses overlap and strongly interfere with each other. This makes the reconstruction and the interpretation of the original signals very difficult.
Therefore, when using T1 lines at lengths of about 6,000 feet, it is necessary to modify the transmitted signal by compensating for the attenuation and smearing in order to be able reconstruct the original transmitted signal. The compensation must supply an amount of gain that is similar to the loss that has been incurred in the T1 line.